1. Field of the Invention
The present invention relates to sensors and more particularly to a sensor which uses the difference in strain and temperature response of fiber Bragg gratings and a long period fiber grating to discriminate between strain and temperature induced wavelength shifts.
2. Description of Related Art
Fiber Bragg gratings (FBGs) are emerging as a new sensor technology for the monitoring and spatial analysis of structural loading. Considerable effort has been expended on the development of fabrication techniques and instrumentation for detecting small wavelength shifts associated with these devices as sensors. One of the remaining technical issues associated with FBG strain sensors is that of thermal apparent strain, which is the inability to distinguish wavelength shifts produced by strain from those produced by temperature.
It is possible to separate strain and temperature by simultaneously measuring the wavelength shift in two gratings which have different responses to strain and temperature. This method depends on having the ratio of strain responses of the two gratings be different from the ratio of temperature responses. One group of scientists have demonstrated this with two FBGs written at 850 and 1300 nanometers (nm), but the ratio of the responses differed by only 15%.
A second group of scientists have reported using an FBG and a long period rocking filter, and observed a large difference in the ratio of responses between the two devices. However, the broadband spectrum of the rocking filter made accurate detection of the wavelength difficult, an effect which the second group overcame by using two rocking filters in a cavity configuration. However, this second group reported large errors of +/-165 .mu.strain in determining strain.
Applicants know of no one in the prior art who has described or demonstrated a sensor which uses the difference in strain and temperature response of fiber Bragg gratings and a long period fiber grating to discriminate between strain and temperature induced wavelength shifts.